Automobile undercarriage part

ABSTRACT

An automobile undercarriage part of the present invention has a welded joint formed by base steel plate,wherein the chemical composition of a weld metal contains, with respect to a total mass of the weld metal, by mass %, C: 0.02% to 0.30%, Si: 0.10% to less than 1.0%, Mn: 1.2% to 3.0%, Al: 0.002% to 0.30%, Ti: 0.005% to 0.30%, P: more than 0% to 0.015%, and S: more than 0% to 0.030%, the following formula (1A), formula (1B), formula (2), and formula (3) are satisfied, and slag in a toe portion of the fillet weld satisfies a formula (4).[Al]+[Ti]&gt;0.05  Formula (1A)[Ti]/[Al]&gt;0.9  Formula(1B)7×[Si]+7×[Mn]−112×[Ti]−30×[Al]≤12  Formula (2)2.0&lt;[Si]+[Mn]  Formula (3)[Ti content on slag surface]&gt;[Si content on slag surface]  Formula (4).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an automobile undercarriage part. The present invention particularly relates to an automobile undercarriage part having a welded joint in which the strength and corrosion resistance of a weld metal are excellent.

Priority is claimed on Japanese Patent Application No. 2019-061002, filed in Japan on Mar. 27, 2019, the content of which is incorporated herein by reference.

RELATED ART

Automobile undercarriage parts are usually manufactured by performing lap welding on a plurality of steel materials through gas shielded arc welding or the like.

The standards for automobile corrosion resistance are becoming stricter year by year, and a need to suppress red rust particularly in joint portions (arc welding portion) in undercarriage parts such as arms, subframes, and beams is intensifying. The parts include a variety of lower arms, a variety of upper arms, toe control arms, trailing arms, torsion beams, carriers, subframes, side rails, cabs, underrun protectors, wheels, and floor cloths.

After these automobile undercarriage parts are manufactured by performing arc welding on a plurality of steel members using welding wires, painting is performed. In this painting, when a painting defect occurs on the surface of a weld metal, there has been a case where not only does the appearance become poor, but also the corrosion resistance deteriorates. In addition, even when automobile undercarriage parts are well painted to look good, if a painting defect such as flaking or exfoliation of paint occurs, there has been a case where red rust occurs between oxide slag on the weld metal and the paint membrane.

Patent Document 1 discloses a gas shielded metal arc welding method that enhances corrosion resistance after painting in a weld or a vicinity thereof, the method being a method of performing gas shielded metal arc welding on a carbon steel base metal that is to be electrodeposition-painted after welding using a steel welding wire, in which a welding wire having a composition that makes the total amount of Si in a base metal and the welding wire reach 0.04% to 0.2% by weight % and makes the total amount of Mn in the base metal and the welding wire reach 0.5% or more is used.

According to this technique of Patent Document 1, it is considered that corrosion resistance after painting in a weld formed by gas shielded metal arc welding or a vicinity thereof can be enhanced by suppressing the formation of insulating Si slag.

PRIOR ART DOCUMENT

[Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H8-33997

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the case of simply reducing Si as in the technique of Patent Document 1, there have been no studies regarding the securement of the strength of weld metal. Therefore, it has been difficult to apply this technique to automobile undercarriage parts that require a high strength of 780 MPa or higher.

The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an automobile undercarriage part having a welded joint in which the strength and corrosion resistance of a weld metal are excellent and which is formed of a base steel plate of 780 MPa or higher.

Means for Solving the Problem

Specific methods of the present invention are as described below.

[1] A first aspect of the present invention is an automobile undercarriage part including a welded joint in which a first steel sheet and a second steel sheet each having a tensile strength of 780 MPa or higher are overlapped and a fillet weld is formed between an end surface of the first steel sheet and a surface of the second steel sheet. A chemical composition of a weld metal that forms the welded joint contains, with respect to a total mass of the weld metal, by mass %, C: 0.02% to 0.30%, Si: 0.10% to less than 1.0%, Mn: 1.2% to 3.0%, Al: 0.002% to 0.30%, Ti: 0.005% to 0.30%, P: more than 0% to 0.015%, S: more than 0% to 0.030%, Cu: 0% to 0.50%, Cr: 0% to 1.5%, Nb: 0% to 0.3%, V: 0% to 0.3%, Mo: 0% to 1.0%, Ni: 0% to 2.5%, B: 0% to 0.005%, and a remainder consisting of iron and impurities, the following formula (1A), formula (1B), formula (2), and formula (3) are satisfied, and slag in a toe portion of the fillet weld satisfies a formula (4).

[Al]+[Ti]>0.05  Formula (1A)

[Ti]/[Al]>0.9  Formula(1B)

7×[Si]+7×[Mn]−112×[Ti]−30×[Al]≤12  Formula (2)

2.0<[Si]+[Mn]  Formula (3)

[Ti content on slag surface]>[Si content on slag surface]  Formula (4).

Here, [Si], [Al], [Ti], and [Mn] mean an amount of each component by mass % with respect to the total mass of the weld metal.

[2] The automobile undercarriage part according to [1], in which the chemical composition of the weld metal may contain, with respect to the total mass of the weld metal, by mass %, one or more of Cu: 0.005% to 0.50%, Cr: 0.05% to 1.5%, Nb: 0.005% to 0.3%, V: 0.005% to 0.3%, Mo: 0.005% to 1.0%, Ni: 0.05% to 2.5%, and B: 0.0005% to 0.005%.

[3] The automobile undercarriage part according to [1] or [2], in which a sheet thickness of the first steel sheet may be 0.8 mm or more and 4.0 mm or less.

Effects of the Invention

According to the undercarriage part according to the present invention, since the composition of the weld metal is appropriately controlled, it is possible to exhibit an excellent strength and excellent corrosion resistance in the weld metal in the welded joint that is formed of the base steel plate of 780 MPa or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an automobile undercarriage part according to the present embodiment.

FIG. 2 is a schematic cross-sectional view showing a welded joint of the automobile undercarriage part according to the present embodiment.

FIG. 3 is a schematic view showing the distance between a nozzle and a base metal.

EMBODIMENTS OF THE INVENTION

As a result of intensive studies regarding measures for solving the above-described problem, the present inventors and the like obtained the following findings.

(A) When the Si content of a weld metal is increased to increase the strength, it is likely that Si-based slag occurs, a painting defect occurs at the Si-based slag occurrence position, and red rust is likely to occur, but the red rust can be suppressed by appropriately controlling the amounts of Si, Mn, Ti, and Al in the weld metal.

(B) In a case where Al and Ti are made to coexist in an appropriately balanced manner in the weld metal, the red rust suppression effect can be obtained.

(C) In a case where Ti and Si on the slag surface are made to coexist in an appropriately balanced manner in a toe portion of a fillet weld in which the slag is likely to gather, the red rust suppression effect can be obtained.

(D) When the nozzle inner diameter is set to 14 to 20 mm, the gas flow rate is set to 20 to 30 L/min, and the distance between a nozzle and a base metal is set to 20 mm or less as welding conditions at the time of forming the fillet weld, it is possible to make Ti and Si on the slag surface coexist in an appropriately balanced manner.

The present invention has been made based on the above-described findings. Hereinafter, an automobile undercarriage part according to an embodiment of the present invention will be described in detail.

The automobile undercarriage part according to the present embodiment is, for example, a variety of lower arms, a variety of upper arms, a toe control arm, a trailing arm, a torsion beam, a carrier, a subframe, a side rail, a cab, an underrun protector, a wheel, and a floor cloth. FIG. 1 is a perspective view of a lower arm which is an automobile undercarriage part, and, as shown in FIG. 1, the automobile undercarriage part according to the present embodiment includes a welded joint 1 at which two steel sheet are overlapped and welded.

FIG. 2 is a schematic cross-sectional view showing the welded joint 1 of the automobile undercarriage part in FIG. 1. As shown in FIG. 2, the welded joint 1 is configured by overlapping two steel sheets 2 and 3 (a first steel sheet and a second steel sheet) and forming a fillet weld 4 between an end surface of one steel sheet 2 and a surface of the other steel sheet 3.

The fillet weld 4 is formed by performing arc welding on the two steel sheets 2 and 3 using a welding wire.

The two steel sheets 2 and 3 may be steel sheets of the same kind or steel sheets of different kinds. The sheet thickness of the steel sheet 2 in which the fillet weld 4 is to be formed on the end surface is preferably 0.8 mm or more and 4.0 mm or less.

When the sheet thickness of the steel sheet 2 is 0.8 mm or more, the occurrence of a welding defect during arc welding is suppressed, and, when the sheet thickness of the steel sheet 2 are 4.0 mm or less, it is possible to suppress an increase in weight. The sheet thickness of the steel sheet 2 is preferably 1.4 mm or more and more preferably 2.0 mm or more. In addition, the sheet thickness of the steel sheet 2 is preferably 3.5 mm or less and more preferably 3.0 mm or less.

It is more preferable that the sheet thicknesses of the steel sheets 2 and 3 are both 0.8 mm or more and 4.0 mm or less.

When the sheet thicknesses of the steel sheets 2 and 3 are 0.8 mm or more, the occurrence of a welding defect during arc welding is suppressed, and, when the sheet thicknesses of the steel sheets 2 and 3 is 4.0 mm or less, it is possible to suppress an increase in weight. The sheet thicknesses of the steel sheets 2 and 3 are preferably 1.4 mm or more and more preferably 2.0 mm or more. In addition, the sheet thicknesses of the steel sheets 2 and 3 are preferably 3.5 mm or less and more preferably 3.0 mm or less.

The composition of the weld metal 4 for the welded joint 1 can be adjusted with the steel sheet component and the welding wire component. Hereinafter, each composition of the weld metal 4 will be described.

In the following description, the expression “corrosion resistance being satisfied” means that no red rust occurs at the time of evaluating the corrosion resistance with 75 cycles of a combined cyclic test (CCT, 5% NaCl, wet rate: 50%) specified in the JASO method M610.

The expression “strength being satisfied” means that, when a tensile test is performed on a welded test piece, the welded test piece fractures from the base metal, not from the weld metal.

The expression “weld metal” means a metal formed by the melting and mixing of a steel sheet base metal and a welding wire.

The chemical composition of the weld metal will be expressed by mass %, which is the proportion in the total mass of the weld metal, and the denotation “mass %” will be simply expressed as “%” in the description.

The chemical composition of the weld metal can be measured by emission spectroscopy with high frequency inductively coupled plasma (ICP). Specifically, (1) a region of the weld metal is specified in advance by visually observing a cross section perpendicular to the longitudinal direction in the longitudinal direction center portion of the weld, (2) the chips of the weld metal are collected by cutting the region with a drill, and (3) the chips are measured as a sample by emission spectroscopy with high frequency inductively coupled plasma (ICP).

[C: 0.02% to 0.30%]

C has an action of stabilizing arc and refining droplets. When the C content is less than 0.02%, droplets become large, arc becomes unstable, and the amount of spatters occurring increases. As a result, bead shapes become uneven and defective, and thus red rust occurs. The reason for the occurrence of red rust due to the bead shape defect is that welding slag is likely to occur in recessed parts attributed to the defect and water, mud containing moisture, or the like, which causes red rust, is likely to gather. In addition, when the C content is less than 0.02%, a tensile strength cannot be obtained in the weld metal, and it is not possible to obtain a desired tensile strength. Therefore, the lower limit of C is 0.02% or more, preferably 0.04% or more, and more preferably 0.06% or more.

On the other hand, when the C content exceeds 0.30%, the weld metal hardens, which degrades cracking resistance and makes the weld metal likely to fracture. Therefore, the upper limit of C is 0.30% and preferably 0.25%.

[Si: 0.10% to Less than 1.0%]

Si is contained in the welding wire or the base metal as a deoxidizing element. In particular, Si in the welding wire accelerates the deoxidation of molten pool and thereby improves the tensile strength of the weld metal. Therefore, the lower limit of Si is 0.10%, preferably 0.20%, and more preferably 0.30%. On the other hand, in a case where Si is excessively contained, the amount of non-conductive Si-based slag increases, and red rust is likely to occur between the slag and the weld metal. Therefore, the upper limit of Si is less than 1.0%, preferably less than 0.9%, and more preferably less than 0.8%. The upper limit is still more preferably 0.7% or less. The upper limit is far still more preferably less than 0.62%.

[Mn: 1.2% to 3.0%]

Mn is also, similar to Si, a deoxidizing element and is an element that accelerates the deoxidation of the molten pool during arc welding and improves the tensile strength of the weld metal. When the Mn content is low, it is not possible to sufficiently secure the tensile strength of the weld metal, and the weld metal is likely to fracture. Therefore, the lower limit of Mn is 1.2% or more and preferably 1.5% or more.

On the other hand, when Mn is excessively contained, the viscosity of molten metal becomes high, and, in a case where the welding rate is fast, it is not possible for the molten metal to appropriately flow into a welded portion, a humping bead is formed, and a bead shape defect is likely to occur. As a result, bead shapes become uneven and defective, and thus red rust occurs. Therefore, the upper limit of Mn is 3.0% or less and preferably 2.5% or less.

[Al: 0.002% to 0.30%]

Al is a strong deoxidizing element and has an effect of accelerating the deoxidation of the molten metal during arc welding to suppress the occurrence of a blowhole. Therefore, the lower limit of the Al content is 0.002%, preferably 0.01%, and more preferably 0.02%.

On the other hand, when the Al content is excessive, the amount of non-conductive Al-based slag increases, and red rust is likely to occur between the slag and the weld metal. Therefore, the upper limit of the Al content of the weld metal is 0.30%, preferably 0.25%, and more preferably 0.20%.

[Ti: 0.005% to 0.30%]

Ti is a deoxidizing element and thus has an effect of suppressing the occurrence of a blowhole. Therefore, the lower limit of the Ti content is 0.005%, preferably 0.01%, and more preferably 0.05%.

On the other hand, in a case where Ti is excessively contained, the amount of Ti-based slag increases, the adhesion between the Ti-based slag and the weld metal deteriorates, and the Ti-based slag is likely to exfoliate. Therefore, red rust is likely to occur at a place where the Ti-based slag has exfoliated. Therefore, the upper limit of the Ti content is 0.30%, preferably 0.25%, and more preferably 0.20%.

[Synergistic Effect of Coexistence of Al and Ti]

Al and Ti are both elements that suppress Si-based slag and form Al-based slag and Ti-based slag and are elements that contribute to the suppression of a painting defect. However, in a case where the amount of only any one of Al and Ti is the lower limit or more, only Al-based slag or only Ti-based slag tends to aggregate on a weld bead. In a case where this slag aggregates, even when there is no painting defect on the weld bead, a cavity is likely to be formed between the weld metal and the slag, and red rust occurs from this cavity. That is, when both Al-based slag and Ti-based slag are formed, the aggregation of the same slag is suppressed, and consequently, red rust is suppressed. Therefore, in the present invention, excellent corrosion resistance can be obtained by providing a component system containing both Al to Ti to the weld metal.

[Al, Ti]

The amounts of Al and Ti satisfy the following formula (1A) and formula (1B).

[Al]+[Ti]>0.05  Formula (1A)

[Ti]/[Al]>0.9  Formula(1B)

Both Al and Ti are capable of sufficiently securing the strength of the weld metal by suppressing the formation of coarse ferrite. In a case where the total amount of Al and Ti is 0.05% or less, even when no blowhole occurs, ferrite in the weld metal is likely to coarsen, the strength of the weld metal can not be sufficiently obtained, and the weld metal is likely to fracture. Therefore, the lower limit of the total amount of Al and Ti is more than 0.05%, preferably 0.10%, and more preferably 0.15%.

The upper limit of Al+Ti is not particularly limited, but is preferably 0.60%, which is calculated from the upper limit of each of Al and Ti. However, when the upper limit of Al+Ti is 0.30% or less, the occurrence of Al-based slag and Ti-based slag is suppressed, and the occurrence of red rust between the Al-based slag and the Ti-based slag and the weld metal is suppressed, which is preferable. The upper limit of Al+Ti is more preferably 0.20%.

In addition, in a component system where the amounts of Si and Mn are large, the balance between Al and Ti is important. Therefore, the amounts of Al and Ti are adjusted so as to satisfy the formula (1B). In a case where this formula (1B) is not satisfied, that is, in a case where Ti/Al is 0.9 or less, the Al-based slag aggregates on the weld bead to increase the thickness, a cavity is likely to be formed between the weld metal and the slag, and there is a concern that red rust may occur from this cavity. The upper limit of the Ti/Al value is not particularly specified.

[Si, Mn, Ti, Al]

In addition, the amounts of Si, Mn, Ti, and Al satisfy the following formula (2).

7×[Si]+7×[Mn]−112×[Ti]−30×[Al]≤12  Formula (2)

As a result of investigating the presence or absence of the occurrence of red rust between slag and a weld metal in weld metals having a variety of component systems, the inventors clarified that, when the value of 7×[Si]+7×[Mn]−112×[Ti]−30×[Al], which is an index regarding the occurrence of red rust, exceeds 12, red rust occurs at an early stage, and corrosion resistance is poor. Therefore, in the above formula, the upper limit is set to 12.

The lower limit is not particularly limited and is −33.5, which is calculated from the lower limits of Si and Mn and the upper limits of Al and Ti.

[Si, Mn]

In addition, the amounts of Si and Mn satisfy the following formula (3).

2.0<[Si]+[Mn]  Formula (3)

When the total amount of Si and Mn is more than 2.0%, it is possible to secure the strength of the weld metal and to prevent the automobile undercarriage part from fracturing from the weld metal when receiving a tensile load.

The upper limit of the amount of Si and Mn is not particularly limited and is less than 4.0%, which is calculated from the upper limits of Si and Mn.

In the above formula, [Si], [Al], [Ti], and [Mn] mean the amount of each component by mass % with respect to the total mass of the weld metal.

Slag in toe portion: [Ti content on slag surface]>[Si content on slag surface]

Since the toe portion of the fillet weld is a place where slag is likely to gather, there is a tendency that red rust is more likely to occur than in any portions other than the toe portion. Therefore, in the fillet weld, slag in the toe portion needs to satisfy the formula (4).

[Ti content on slag surface]>[Si content on slag surface]  Formula (4).

In a case where the slag in the toe portion of the fillet weld does not satisfy the formula (4), insulating Si-based slag aggregates on the weld bead to increase the thickness, a cavity is likely to be formed between the weld metal and the slag, and there is a concern that red rust may occur from this cavity.

The Ti content and the Si content on the slag surface can be measured with a scanning electron microscope (SEM) and an energy dispersive X-ray spectrometer (EDS).

More specifically, the fillet weld is collected by cutting and embedded in a resin. After that, the sample embedded in the resin is polished to a place where there is no slag exfoliation. The slag-attached portion in the cross section of the weld is enlarged and observed with the SEM, and the average Si concentration and the average Ti concentration in a 100 μm×100 μm region of the slag are obtained with the EDS. In order to prevent observation point-dependent variation, the average of the observation results from three cross sections is adopted as each of the average Si concentration and the average Ti concentration.

[P: More than 0% to 0.015%]

P is an element that is usually contained in steel by accident as an impurity and ordinarily contained in steel sheets and welding wires and is thus also contained in weld metals. Here, P is one of key elements that cause the hot cracking of weld metals and is thus desirably suppressed as much as possible. When the P content exceeds 0.015%, since the hot cracking of the weld metal becomes significant, the upper limit of the P content of the weld metal is 0.015% or less.

The lower limit of the P content is not particularly limited and is thus more than 0%, but may be 0.001% from the viewpoint of the dephosphorization cost and the productivity.

[S: More than 0% to 0.030%]

S is also, similar to P, an element that is usually contained in steel by accident as an impurity and ordinarily contained in welding wires and is thus also contained in weld metals. Here, S is an element that impairs the cracking resistance of weld metals and is preferably suppressed as much as possible. When the S content exceeds 0.030%, since the cracking resistance of the weld metal deteriorates, the S content of the weld metal is 0.030% or less.

The lower limit of the S content is not particularly limited and is thus more than 0%, but may be 0.001% from the viewpoint of the desulfurization cost and the productivity.

Cu, Cr, Nb, V, Mo, Ni, and B are not essential elements, and one or more thereof may be contained at the same time as necessary. An effect that can be obtained by containing each element and the upper limit will be described. In a case where these elements are not contained, the lower limit is 0%.

[Cu: 0% to 0.50%]

Since there are cases where Cu derives from Cu plates on welding wires and is contained in weld metals, 0.005% or more of Cu may be contained. On the other hand, when the Cu content becomes excessive, since weld cracking is likely to occur, the upper limit of Cu is 0.50% or less.

[Cr: 0% to 1.5%]

0.05% or more of Cr may be contained in order to enhance the hardenability of the weld and improve the tensile strength. On the other hand, when Cr is excessively contained, the elongation of the weld deteriorates. Therefore, the upper limit of Cr is 1.5% or less.

[Nb: 0% to 0.3%]

0.005% or more of Nb may be contained in order to enhance the hardenability of the weld and improve the tensile strength. On the other hand, when Nb is excessively contained, the elongation of the weld deteriorates. Therefore, the upper limit of Nb is 0.3% or less.

[V: 0% to 0.3%]

0.005% or more of V may be contained in order to enhance the hardenability of the weld and improve the tensile strength. On the other hand, when V is excessively contained, the elongation of the weld deteriorates. Therefore, the upper limit of V is 0.3% or less.

[Mo: 0% to 1.0%]

0.005% or more of Mo may be contained in order to enhance the hardenability of the weld and improve the tensile strength. On the other hand, when Mo is excessively contained, the elongation of the weld deteriorates. Therefore, the upper limit of Mo is 1.0% or less.

[Ni: 0% to 2.5%]

0.05% or more of Ni may be contained in order to improve the tensile strength and elongation of the weld. On the other hand, in a case where Ni is excessively contained, weld cracking is likely to occur. Therefore, the upper limit of Ni is 2.5% or less. The upper limit is preferably 2.0% or less.

[B: 0% to 0.005%]

0.0005% or more of B may be contained in order to enhance the hardenability of the weld and improve the tensile strength. On the other hand, when B is excessively contained, the elongation of the weld deteriorates. Therefore, the upper limit of B is 0.005%. The upper limit is preferably 0.003% or less.

The remainder of the components described above consists of Fe and impurities. The impurities refer to components that are contained in raw materials or components that are contained by accident in manufacturing processes that are not components intentionally contained in the weld metal or components that are permitted to an extent that the automobile undercarriage part according to the present embodiment is not adversely affected.

Hitherto, the automobile undercarriage part according to the present embodiment has been described. The kind of a steel sheet that forms the base metal for the automobile undercarriage part according to the present embodiment is not particularly limited as long as the tensile strength is 780 MPa or higher, but is preferably a steel sheet containing components of C: 0.030% to 0.40%, Si: 0.4% to 1.8%, Mn: 1.80% to 3.20%, P: less than 0.05%, and S: less than 0.010%. In addition, the steel sheet may be a steel sheet containing, in addition to the above-described components, an optional component such as Al or Ti.

Next, a manufacturing method of the automobile undercarriage part according to the present embodiment will be described. Hereinafter, a case where the fillet weld is formed by gas shielded arc welding will be described as an example.

Nozzle inner diameter of 14 to 20 mm, gas flow rate of 20 to 30 L/min, and distance between nozzle and base metal of 20 mm or less.

In the manufacturing of the automobile undercarriage part according to the present embodiment, as welding conditions at the time of forming the fillet weld by gas shielded arc welding, the nozzle inner diameter is set to 14 to 20 mm, the gas flow rate is set to 20 to 30 L/min, and the distance between the nozzle and the base metal is set to 20 mm or less. In such a case, the weld metal is less likely to be oxidized, and [Ti content on slag surface]>[Si content on slag surface] is satisfied in the toe portion of the fillet weld.

As shown in FIG. 3, a distance 6 between the nozzle and the base metal is the shortest distance between the tip end portion of the outlet for a shielding gas 52 in a welding nozzle 51 and an aiming position 7 of a welding wire 53. FIG. 3 is a view showing a case where the aiming position 7 of the welding wire 53 is an intersection portion 54 between a plane that passes through the end surface of the steel sheet 2 and the surface of the steel sheet 3.

The manufacturing method of the automobile undercarriage part according to the present embodiment more preferably satisfies the formula (5).

(10/6×[nozzle inner diameter]−20/6)×0.8≤[gas flow rate]≤(10/6×[nozzle inner diameter]−20/6)×1.2  Formula (5)

When the formula (5) is satisfied, the shielding state of the weld metal surface becomes more favorable, and it becomes more difficult for the weld metal to be oxidized. Therefore, it becomes easy to control the amounts of Ti and Si on the slag surface in the toe portion of the fillet weld to the relationship of [Ti content on slag surface]>[Si content on slag surface].

In the manufacturing method of the automobile undercarriage part according to the present embodiment, the inclination angle (raising angle) of a welding torch is more preferably 50° to 70°. In a case where the inclination angle (raising angle) of the welding torch is 50° or more, the shielding state of the weld metal surface becomes more favorable, and it becomes more difficult for the weld metal to be oxidized. Therefore, it becomes easy to control the amounts of Ti and Si on the slag surface in the toe portion of the fillet weld to the relationship of [Ti content on slag surface]>[Si content on slag surface]. On the other hand, when the inclination angle (raising angle) of the welding torch is 70° or less, it is possible to favorably hold the penetration shape of the weld metal.

The inclination angle (raising angle) of the welding torch is an acute angle formed by the longitudinal direction of the torch and the surface of the steel sheet 3.

Examples

Hereinafter, the effect of the present invention will be specifically described using examples.

Lap fillet arc welding was performed using a variety of welding wires on steel sheets having a tensile strength of 980 MPa or on steel sheets having a tensile strength of 780 MPa to manufacture welded joints, and weld metals were evaluated. One and the other steel sheets were caused to adhere to each other in an overlap space set to 15 mm, and the sheet thickness of the steel sheet was set to 2.6 mm. Regarding the welding position, a weld line was set to be horizontal, and the inclination angle α of the other steel sheet was set to 0°. As the welding method, a pulse MAG arc welding method was adopted, the inclination angle (raising angle) of the welding torch was set to 60°, and Ar gas containing 20 vol % of CO₂ was mainly used as the shielding gas. In addition, as the shielding gas, Ar gas containing 3% of O₂ and Ar gas containing 20% of CO₂ and 2% of O₂ were also used. The wire tip aim was the corner portion that was configured by the end surface of the one steel sheet and the surface of the other steel sheet.

In addition, as welding conditions for Experiment Nos. 1 to 28 and Nos. 29 to 42, the nozzle inner diameter was set to 14 to 20 mm, the gas flow rate was set to 20 to 30 L/min, and the distance between the nozzle and the base metal was set to 20 mm or less. Furthermore, the relationship between the nozzle inner diameter and the gas flow rate satisfied the formula (5), and the inclination angle (raising angle) of the welding torch was 50° to 70°.

The components of the weld metals were adjusted by using solid wires of a variety of component systems as the welding wires.

The tensile strength, sheet thickness, and main components of each steel sheet are as described below.

(980 MPa steel sheet)

-   -   Sheet thickness: 2.6 mm     -   Components: C=0.060%, Si=1.2%, Mn=2.50%, P=0.01%, S=0.005%,         Al=0.03%, and Ti=0.12%

(780 MPa steel sheet)

-   -   Sheet thickness: 2.6 mm     -   Components: C=0.045%, Si=0.02%, Mn=1.55%, P=0.01%, S=0.005%,         Al=0.3%, and Ti=0.13%

For the welded joints obtained as described above, the chemical compositions of the weld metals were measured.

Specifically, the chemical composition of the weld metal was measured by (1) specifying a region of the weld metal in advance by visually observing a cross section perpendicular to the longitudinal direction in the longitudinal direction center portion of the weld, (2) collecting the chips of the weld metal by cutting the region with a drill, and (3) measuring the chips as a sample by emission spectroscopy with high frequency inductively coupled plasma (ICP).

Table 1, Table 2, and Table 3 show the amount of each component and the values of the formula (1A), the formula (1B), the formula (2), and the formula (3). Numerical values outside the scope of the present invention are underlined. In addition, for components that were not added, cells in the table were left blank.

In addition, the Ti content and the Si content on the slag surface in the toe portion of the fillet weld were measured with SEM-EDS by the above-described method. The Ti content and the Si content on the slag surface in the toe portion of the fillet weld were regarded as OK in the case of satisfying the formula (4) and regarded as NG in the case of not satisfying the formula (4). Table 4 also shows the strength (fracturing position), the determination result based on the formula (4), and the evaluation result of red rust for each experimental example.

TABLE 1 Experiment Steel C Si Mn Al Ti P S Cu Cr No. type (mass %) Remainder being Fe and impurity 1 980 0.08 0.48 2.0 0.01 0.08 0.009 0.007 2 980 0.08 0.61 2.0 0.01 0.07 0.007 0.004 3 980 0.08 0.68 2.2 0.01 0.08 0.009 0.004 4 980 0.08 0.37 1.8 0.10 0.11 0.010 0.008 0.006 5 980 0.08 0.45 1.6 0.10 0.12 0.004 0.009 6 980 0.07 0.50 2.0 0.06 0.07 0.009 0.015 7 980 0.07 0.59 1.8 0.05 0.07 0.010 0.004 8 980 0.07 0.71 2.8 0.08 0.29 0.006 0.006 9 980 0.07 0.40 3.0 0.03 0.14 0.009 0.011 10 980 0.07 0.46 1.6 0.05 0.15 0.008 0.005 11 980 0.09 0.52 1.8 0.01 0.17 0.011 0.018 12 980 0.09 0.63 2.0 0.09 0.10 0.011 0.004 13 980 0.09 0.79 1.3 0.03 0.08 0.004 0.009 14 980 0.09 0.45 1.6 0.06 0.07 0.009 0.005 0.012 Experiment Nb V Mo Ni B Formula Formula Formula Formula No. (mass %) Remainder being Fe and impurity (1A) (1B) (2) (3) Classification 1 0.09 8.00 8.1 2.48 Invention Example 2 0.08 7.00 10.1 2.61 Invention Example 3 0.03 0.09 8.00 10.9 2.88 Invention Example 4 0.21 1.10 −0.1 2.17 Invention Example 5 0.22 1.20 −2.1 2.05 Invention Example 6 0.08 0.13 1.17 7.9 2.50 Invention Example 7 0.12 1.40 7.4 2.39 Invention Example 8 0.37 3.63 −10.3 3.51 Invention Example 9 0.17 4.67 7.2 3.40 Invention Example 10 0.002 0.20 3.00 −3.9 2.06 Invention Example 11 0.18 17.00 −3.1 2.32 Invention Example 12 0.01 0.19 1.11 4.5 2.63 Invention Example 13 0.11 2.67 4.8 2.09 Invention Example 14 0.06 0.13 1.17 4.7 2.05 Invention Example

TABLE 2 Experiment Steel C Si Mn Al Ti P S Cu Cr No. type (mass %) Remainder being Fe and impurity 15 980 0.09 0.64 1.4 0.08 0.17 0.009 0.015 16 980 0.06 0.50 2.5 0.10 0.12 0.009 0.011 0.07 17 980 0.06 0.62 1.9 0.10 0.20 0.007 0.008 18 980 0.06 0.71 1.3 0.10 0.12 0.009 0.005 0.07 19 980 0.06 0.37 3.0 0.10 0.20 0.007 0.006 20 980 0.06 0.46 2.0 0.10 0.20 0.007 0.007 0.009 21 980 0.03 0.59 1.9 0.10 0.24 0.007 0.008 22 980 0.12 0.62 1.4 0.10 0.12 0.009 0.005 0.07 23 980 0.25 0.38 2.0 0.10 0.20 0.007 0.008 24 980 0.28 0.47 1.6 0.10 0.22 0.007 0.008 0.015 25 780 0.07 0.63 1.4 0.10 0.12 0.007 0.008 26 780 0.08 0.76 1.6 0.10 0.22 0.007 0.008 27 780 0.06 0.52 2.2 0.09 0.19 0.007 0.008 28 780 0.09 0.78 1.3 0.08 0.21 0.007 0.008 Experiment Nb V Mo Ni B Formula Formula Formula Formula No. (mass %) Remainder being Fe and impurity (1A) (1B) (2) (3) Classification 15 0.25 2.13 −7.2 2.04 Invention Example 16 0.22 1.20 4.6 3.00 Invention Example 17 0.30 2.00 −7.8 2.52 Invention Example 18 0.22 1.20 −2.4 2.01 Invention Example 19 0.30 2.00 −1.8 3.37 Invention Example 20 0.30 2.00 −8.2 2.46 Invention Example 21 0.34 2.40 −12.5 2.49 Invention Example 22 0.22 1.20 −2.3 2.02 Invention Example 23 0.30 2.00 −8.7 2.38 Invention Example 24 0.32 2.20 −13.2 2.07 Invention Example 25 0.22 1.20 −2.2 2.03 Invention Example 26 0.32 2.20 −11.1 2.36 Invention Example 27 0.28 2.11 −4.9 2.72 Invention Example 28 0.08 0.29 2.63 −11.4 2.08 Invention Example

TABLE 3 Experiment Steel C Si Mn Al Ti P S Cu Cr No. type (mass %) Remainder being Fe and impurity 29 980 0.08 1.21 2.5 0.05 0.05 0.009 0.004 30 980 0.08 0.72 2.5 0.05 0.05 0.009 0.005 31 980 0.32 0.91 1.2 0.07 0.08 0.009 0.015 32 980 0.07 0.04 2.0 0.01 0.03 0.008 0.011 33 980 0.01 0.05 2.1 0.06 0.07 0.009 0.005 34 980 0.07 0.07 3.5 0.03 0.14 0.008 0.011 0.015 35 980 0.07 0.78 1.5 0.32 0.30 0.005 0.005 36 980 0.08 0.06 2.0 0.29  0.000 0.011 0.018 37 980 0.09 0.05 0.2 0.05 0.07 0.007 0.005 38 980 0.08 0.05 2.0 0.05 0.34 0.005 0.004 39 980 0.08 0.01 2.5  0.000 0.11 0.009 0.005 0.009 40 980 0.07 0.59 1.2 0.05 0.07 0.007 0.005 41 980 0.07 0.71 2.2 0.06 0.05 0.005 0.004 42 780 0.08 1.34 2.5 0.05 0.05 0.008 0.002 43 980 0.08 0.95 1.2 0.01 0.05 0.007 0.004 Experiment Nb V Mo Ni B Formula Formula Formula Formula No. (mass %) Remainder being Fe and impurity (1A) (1B) (2) (3) Classification 29 0.10 1.00 18.9 3.71 Comparative Example 30 0.10 1.00 15.4 3.22 Comparative Example 31 0.15 1.14  3.7 2.11 Comparative Example 32 0.04 3.00 10.6 2.04 Comparative Example 33 0.13 1.17  5.4 2.15 Comparative Example 34 0.17 4.67  8.4 3.57 Comparative Example 35 0.62 0.94 −27.2   2.28 Comparative Example 36 0.29 —  5.7 2.06 Comparative Example 37 0.12 1.40  −7.6   0.25 Comparative Example 38 0.39 6.80 −25.2   2.05 Comparative Example 39 0.11 —  5.3 2.51 Comparative Example 40 0.12 1.40  3.2 1.79 Comparative Example 41 0.11 0.83 13.0 2.91 Comparative Example 42 0.10 1.00 19.8 3.84 Comparative Example 43 0.06 5.00  9.2 2.15 Comparative Example

TABLE 4 Experiment Fracturing Red rust Red rust Red rust No. position Formula (4) 50 cycles 75 cycles 100 cycles Classification 1 OK OK OK OK OK Invention Example 2 OK OK OK OK OK Invention Example 3 OK OK OK OK OK Invention Example 4 OK OK OK OK OK Invention Example 5 OK OK OK OK OK Invention Example 6 OK OK OK OK OK Invention Example 7 OK OK OK OK OK Invention Example 8 OK OK OK OK NG Invention Example 9 OK OK OK OK OK Invention Example 10 OK OK OK OK OK Invention Example 11 OK OK OK OK OK Invention Example 12 OK OK OK OK OK Invention Example 13 OK OK OK OK NG Invention Example 14 OK OK OK OK OK Invention Example 15 OK OK OK OK OK Invention Example 16 OK OK OK OK OK Invention Example 17 OK OK OK OK OK Invention Example 18 OK OK OK OK NG Invention Example 19 OK OK OK OK OK Invention Example 20 OK OK OK OK OK Invention Example 21 OK OK OK OK OK Invention Example 22 OK OK OK OK OK Invention Example 23 OK OK OK OK OK Invention Example 24 OK OK OK OK OK Invention Example 25 OK OK OK OK OK Invention Example 26 OK OK OK OK NG Invention Example 27 OK OK OK OK OK Invention Example 28 OK OK OK OK NG Invention Example 29 OK NG NG NG NG Comparative Example 30 OK NG NG NG NG Comparative Example 31 NG OK OK OK OK Comparative Example 32 NG NG OK NG NG Comparative Example 33 NG OK NG NG NG Comparative Example 34 OK OK NG NG NG Comparative Example 35 OK OK NG NG NG Comparative Example 36 OK NG NG NG NG Comparative Example 37 NG OK OK OK OK Comparative Example 38 OK OK NG NG NG Comparative Example 39 OK NG NG NG NG Comparative Example 40 NG OK OK OK OK Comparative Example 41 OK NG NG NG NG Comparative Example 42 OK NG NG NG NG Comparative Example 43 OK NG OK NG NG Comparative Example

(Evaluation of Strength)

The strength was evaluated at the fracturing position in a joint tensile test. The tensile test was performed by overlapping the end portions in the longitudinal direction of two 25 mm×100 mm steel sheets as much as 15 mm, performing lap fillet welding, and pulling the steel sheets at a rate of a tensile rate of 10 mm/min in the longitudinal direction. The strength was evaluated as OK in a case where the fracturing position was the base metal and evaluated as NG in a case where the fracturing position was the weld metal.

(Evaluation of Red Rust)

The combined cyclic test (CCT, 5% NaCl, wet rate: 50%) specified in the JASO method M610 was performed 50 cycles, 75 cycles, and 100 cycles, and the presence or absence of red rust was evaluated. A case where red rust did not occur was regarded as OK, and a case where red rust occurred was regarded as NG.

Corrosion resistance was evaluated into the following four grades A, B, C, and D. A and B indicate that the corrosion resistance was satisfied, and C and D indicate that the corrosion resistance was not satisfied.

A: All of the 50-cycle test, the 75-cycle test, and the 100-cycle test are OK.

B: The 50-cycle test and the 75-cycle test are OK, but the 100-cycle test is NG.

C: The 50-cycle test is OK, but the 75-cycle test and the 100-cycle test are NG.

D: All of the 50-cycle test, the 75-cycle test, and the 100-cycle test are NG.

In Experiment Nos. 1 to 28 according to the present invention examples, since the compositions of the weld metals and the welding conditions were appropriate, and thus the compositions of the weld metals and the compositions of the slag surfaces in the toe portions of the fillet welds were appropriately controlled, it was possible to obtain an excellent strength and excellent corrosion resistance in the weld metals. However, in Experiment Nos. 8, 13, 18, 26, and 28 in which the Si content was more than 0.7%, in the combined cyclic tests, red rust did not occur in the 50-cycle test and the 75-cycle test, but red rust occurred in the 100-cycle test.

In Experiment No. 29 according to a comparative example, since the Si content of the weld metal was excessive and the formula (4) was not satisfied, non-conductive slag was formed, and red rust occurred.

In Experiment No. 30 according to a comparative example, since the chemical composition of the weld metal did not satisfy the formula (2) and the formula (4), it was not possible to suppress the occurrence of red rust.

In Experiment No. 31 according to a comparative example, since the C content of the weld metal was excessive, the weld metal hardened, and it was not possible to obtain a desired strength.

In Experiment No. 32 according to a comparative example, since the Si content of the weld metal was too low, and the chemical composition of the weld metal did not satisfy the formula (1), it was not possible to sufficiently benefit the effect of suppressing the formation of coarse ferrite, and it was not possible to secure the strength of the weld metal. In addition, due to the fact that the formula (4) was not satisfied, red rust occurred after 75 cycles and 100 cycles of the combined cyclic test.

In Experiment No. 33 according to a comparative example, since the C content of the weld metal was too low, it was not possible to obtain a tensile strength in the weld metal, and it was not possible to obtain a desired tensile strength. In addition, due to a bead shape defect, red rust occurred.

In Experiment No. 34 according to a comparative example, since the Mn content of the weld metal was excessive, due to a bead shape defect, red rust occurred.

In Experiment No. 35 according to a comparative example, since the Al content of the weld metal was excessive, Al-based slag increased, and red rust occurred between the Al-based slag and the weld metal.

In Experiment No. 36 according to a comparative example, since the Ti content of the weld metal was 0%, and Al and Ti did not coexist, only Al-based slag aggregated on the weld bead, a cavity was formed between the weld metal and the Al-based slag, and red rust occurred from this cavity. In addition, due to the fact that the formula (4) was not satisfied, red rust occurred.

In Experiment No. 37 according to a comparative example, since the Mn content of the weld metal was too low, it was not possible to ensure the strength of the weld metal, and fracturing occurred in the weld metal.

In Experiment No. 38 according to a comparative example, since the Ti content of the weld metal was excessive, Ti-based slag increased, the adhesion of the Ti-based slag deteriorated, and the Ti-based slag exfoliated. Therefore, red rust occurred at a place where the Ti-based slag had exfoliated.

In Experiment No. 39 according to a comparative example, since the Al content of the weld metal was 0%, and Al and Ti did not coexist, only Ti-based slag aggregated on the weld bead, a cavity was formed between the weld metal and the Ti-based slag, and red rust occurred from this cavity. In addition, due to the fact that the formula (4) was not satisfied, red rust occurred.

In Experiment No. 40 according to a comparative example, since the chemical composition of the weld metal did not satisfy the formula (3), it was not possible to secure the strength of the weld metal, and fracturing occurred from the weld metal when a tensile load was applied.

In Experiment No. 41 according to a comparative example, since the chemical composition of the weld metal did not satisfy the formula (1B), red rust occurred due to the fact that Al-based slag aggregated and the formula (4) was not satisfied.

In Experiment No. 42 according to a comparative example, since the Si content of the weld metal was excessive and the formula (4) was not satisfied, non-conductive slag was formed, and red rust occurred.

In Experiment No. 43 according to a comparative example, regarding the welding conditions, the nozzle inner diameter was smaller than 14 mm, the gas flow rate was slower than 20 L/min, and the distance between the nozzle and the base metal was longer than 20 mm. As a result, since the formula (4) was not satisfied, red rust occurred after 75 cycles and 100 cycles of the combined cyclic test.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide an automobile undercarriage part having a welded joint in which the strength and corrosion resistance of a weld metal are excellent and which is formed of a base steel plate of 780 MPa or higher, and the industrial applicability is high.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   1 Welded joint     -   2 First steel sheet     -   3 Second steel sheet     -   4 Fillet weld, weld metal     -   51 Nozzle (welding torch)     -   52 Shielding gas     -   53 Welding wire     -   54 intersection portion     -   6 Distance between nozzle and base metal     -   7 Aiming position 

1. An automobile undercarriage part comprising: a welded joint in which a first steel sheet and a second steel sheet each having a tensile strength of 780 MPa or higher are overlapped and a fillet weld is formed between an end surface of the first steel sheet and a surface of the second steel sheet, wherein a chemical composition of a weld metal that forms the welded joint contains, with respect to a total mass of the weld metal, by mass %: C: 0.02% to 0.30%; Si: 0.10% to less than 1.0%; Mn: 1.2% to 3.0%; Al: 0.002% to 0.30%; Ti: 0.005% to 0.30%; P: more than 0% to 0.015%; S: more than 0% to 0.030%; Cu: 0% to 0.50%; Cr: 0% to 1.5%; Nb: 0% to 0.3%; V: 0% to 0.3%; Mo: 0% to 1.0%; Ni: 0% to 2.5%; B: 0% to 0.005%; and a remainder consisting of iron and impurities, the following formula (1A), formula (1B), formula (2), and formula (3) are satisfied, and slag in a toe portion of the fillet weld satisfies the formula (4), [Al]+[Ti]>0.05  Formula (1A) [Ti]/[Al]>0.9  Formula(1B) 7×[Si]+7×[Mn]−112×[Ti]−30×[Al]≤12  Formula (2) 2.0<[Si]+[Mn]  Formula (3) [Ti content on slag surface]>[Si content on slag surface]  Formula (4). where [Si], [Al], [Ti], and [Mn] mean an amount of each component by mass % with respect to the total mass of the weld metal.
 2. The automobile undercarriage part according to claim 1, wherein the chemical composition of the weld metal contains, with respect to the total mass of the weld metal, by mass %, one or more of: Cu: 0.005% to 0.50%; Cr: 0.05% to 1.5%; Nb: 0.005% to 0.3%; V: 0.005% to 0.3%; Mo: 0.005% to 1.0%; Ni: 0.05% to 2.5%; and B: 0.0005% to 0.005%.
 3. The automobile undercarriage part according to claim 1, wherein a sheet thickness of the first steel sheet is 0.8 mm or more and 4.0 mm or less.
 4. An automobile undercarriage part comprising: a welded joint in which a first steel sheet and a second steel sheet each having a tensile strength of 780 MPa or higher are overlapped and a fillet weld is formed between an end surface of the first steel sheet and a surface of the second steel sheet, wherein a chemical composition of a weld metal that forms the welded joint contains, with respect to a total mass of the weld metal, by mass %: C: 0.02% to 0.30%; Si: 0.10% to less than 1.0%; Mn: 1.2% to 3.0%; Al: 0.002% to 0.30%; Ti: 0.005% to 0.30%; P: more than 0% to 0.015%; S: more than 0% to 0.030%; Cu: 0% to 0.50%; Cr: 0% to 1.5%; Nb: 0% to 0.3%; V: 0% to 0.3%; Mo: 0% to 1.0%; Ni: 0% to 2.5%; B: 0% to 0.005%; and a remainder comprising iron and impurities, the following formula (1A), formula (1B), formula (2), and formula (3) are satisfied, and slag in a toe portion of the fillet weld satisfies the formula (4), [Al]+[Ti]>0.05  Formula (1A) [Ti]/[Al]>0.9  Formula(1B) 7×[Si]+7×[Mn]−112×[Ti]−30×[Al]≤12  Formula (2) 2.0<[Si]+[Mn]  Formula (3) [Ti content on slag surface]>[Si content on slag surface]  Formula (4). where [Si], [Al], [Ti], and [Mn] mean an amount of each component by mass % with respect to the total mass of the weld metal. 